1. Field of the Invention
The present invention relates to a Raman amplification method of Raman-amplifying signal light of a plurality of channels which are included in a predetermined wavelength band and have different wavelengths, a Raman amplification pumping unit for supplying pumping light of a plurality of channels having different wavelengths to an optical transmission path through which the signal light of the plurality of channels propagates, and a WDM optical communication system to which the Raman amplification pumping unit is applied.
2. Related Background Art
In a general optical communication system, the signal light sent out from a transmitter will decrease in power by the time it reaches a receiver because of the transmission loss caused when it is transmitted through an optical transmission path. If the power of the signal light having reached the receiver is lower than a predetermined value, optical communication may not be properly performed due to a reception error. For this reason, an optical amplifier is placed on the optical transmission path between the transmitter and the receiver. Signal light is optically amplified by this optical amplifier to compensate for the transmission loss in the signal light on the optical transmission path.
In a WDM (Wavelength Division Multiplexing) optical communication system using multiplexed signal light of a plurality channels which are included in a predetermined wavelength band and have different wavelengths, it is important to flatten the gain spectrum of an optical amplifier in the predetermined wavelength band (signal wavelength band). This is because if the gain spectrum is not flat, although signal light of a given channel of the plurality of channels is properly received by a receiver, signal light of another channel may not be properly received because of a reception error. In Raman amplification, a gain spectrum is flattened by appropriately setting the pumping light powers of a plurality of channels supplied to an optical transmission path, especially the powers of the respective pumping channels (having different wavelengths).
In practice, however, the transmission loss in signal light changes due to the influences of the bending of the optical transmission path, ambient temperature, and the like. For this reason, each pumping light power must be adjusted appropriately in accordance with loss changes in signal light.
For example, according to Japanese Patent Laid-Open No. 10-150414, one of a plurality of amplified signal channels which has the maximum power is selected, and a pumping light power is so adjusted as to set the optical power of the selected signal channel to a predetermined value.